The isolated rabbit tubule preparation will be developed as a model for acid secretion. Using O2 consumption, aminopyrine uptake, pH indicator dyes and morphological changes the role of cAMP and the cyclase systems will be investigated in stimulation. With spectroscopic and metabolite measurement the role of intermediary metabolism in maintaining H ion secretion will be determined. Membranes isolated from inhibited and stimulated tubules will be compared as to composition and transport properties using dyes, radioactive ions and electrode techniques.